I come with some good news, I have fixed the issues with the dialup line
and it appears that all is now well.

Since it is on a Vonage connection, I've limited the modem to negotiate
at 2400 baud, if there is still issues, I will look in to reducing it
lower.

I have also changed the way it connects in the backend, there is now a
secure connection in to the private network in the Google Cloud data
centre, therefore no longer spewing out data over the net unencrypted.